The invention relates to the field of construction vehicles and to detection of a laser reference plane in order to permit accurate and controlled positioning of a tool carried by a construction vehicle, the tool being carried at a predetermined distance from the laser reference plane as the vehicle moves along a land surface.
In many fields of construction, such as earth grading, road construction, ditch digging, curb laying and the like, it has been recognized that a reference line or plane is needed to accurately lay out the route or elevation of the road, curb or ditch. In recent years the desirability of using a laser reference plane positioned above the working vechicle has been appreciated, and apparatuses have been evolved to utilize such a plane in the guidance of a tool carrying vehicle.
One of the principal uses for such a laser reference plane is for land grading where a road grader or the like must level substantial areas of land for roads, new construction, parking lots, air fields or the like. It is known to equip an earth grader with a laser beam detector and to have the detector automatically follow a laser reference plane. It is also known to raise and lower the grader blade as the detector follows the laser plane in order to keep the blade a predetermined distance from the detector. Using these techniques, prior art devices have endeavored to keep the grader blade at approximately the predetermined distance below the laser reference plane and to thereby closely control the elevation of the land being worked.
The known detector apparatuses for following the laser reference plane utilize a pair of vertically positioned photocells with upper and lower cells spaced apart to receive and keep the laser plane therebetween. The photocells have been mounted on a planar surface and face in the same single direction. For best reception of the laser beam, the detector must be oriented with the plane of the detector roughly perpendicular to the source of laser light so the photocells confront the laser beam source. While it is possible to receive the laser beam with the photocells even when the photocells do not directly confront the laser beam, in practice the arc through which the photocell may successfully receive and detect a laser beam is approximately 100.degree.. Accordingly, if the laser beam source is outside this arc, the detector will not respond to or detect the laser reference plane. Accordingly, the known detectors must be kept in fairly directly confronting relationship with the laser source and continually manipulated to face the laser source each time the vehicle turns. It can be readily appreciated that a construction vehicle moving over an irregular land surface is frequently turning and accordingly it is difficult and inconvenient to keep the known detectors in continuous confronting relationship with the laser source. It would be desirable to provide a laser beam detector which is responsive over a substantially larger arc and preferably through 360.degree. to thereby eliminate the risk of losing contact between the laser beam source and the detector due to the detector facing in the wrong direction.
The available detectors have the upper and lower photosensitive cells closely spaced at a fixed, unvarying distance apart, and the object is to keep the laser reference plane positioned between these upper and lower photocells. This object can be a very difficult one during relatively coarse grading when the road grader moves over uneven ground with attendant bouncing and pitching movement. Under such conditions the detector can suddenly shift above or below the laser reference plane and easily lose the laser plane. When such loss occurs, the detector must be moved upwardly or downwardly to pursue and search for the laser reference plane. When the vehicle is frequently bouncing and pitching, the searching action of the detector can be substantial, and considerable inaccuracy in the grade level can result when the detector is out of contact with the laser plane.
Because the width of laser beams increases linearly with distance from the laser source, the beam width and accordingly the laser reference plane thickness increase with distance from the laser source. As a result, it is desirable to have the upper and lower photocells positioned at a greater distance apart when grading is done a substantial distance from the laser source and to have the cells more closely spaced for projects near the laser source. The existing detectors are less effective when used at substantial distances from the laser source because the laser plane thickness is greater and the known detectors cannot vary the distance separating the photocells. At greater distances the laser plane can be thick enough to continually impinge on both upper and lower photocells of known detectors, and such impingement provides the known detectors with contradictory and unusable information because the detector would erroneously be led to move both upwardly and downwardly to follow the laser plane.
It is desirable to provide a detection apparatus which can be used at all working distances from the laser source and be equally adaptable for both fine and coarse grading, with minimal problems of following the laser reference plane under likely-to-be encountered land surface conditions. The present invention meets these needs and provides a greatly improved apparatus by which tool position can be carefully controlled relative to the laser reference plane.